1. Field
The present disclosure relates to redox flow batteries, and more particularly, to redox flow batteries having high energy densities and charge/discharge efficiencies.
2. Description of the Related Art
Secondary batteries are high-efficiency storage systems and are used in various applications from small mobile phones to middle and large power storage units. In particular, the secondary battery is used as the main component in semiconductor and liquid crystal applications, audio and video applications, and information communication applications such as mobile phones and laptops and, recently, as power sources in hybrid electric vehicles.
Such power storage systems are required to have stable energy supply and high energy conversion efficiency. Recently, there has been interest in redox flow batteries as secondary batteries having high output power and high durability that are most suitable for large-sized power storage systems.
Unlike other batteries, the active materials in the redox flow battery exist as ions in an aqueous solution state instead of a solid state, and the redox flow battery operates by a mechanism that stores and generates electric energy due to oxidation/reduction reactions of the ions in the positive and negative electrodes.
That is, the redox flow battery contains an electrolyte (solution) state, in which the active materials of the electrode are dissolved in a solvent. When a battery including a catholyte (electrolyte adjacent to the cathode in an electrochemical cell) and an anolyte (electrolyte adjacent to the anode in an electrochemical cell), each having different oxidation numbers, is charged, an oxidation reaction and a reduction reaction occur in the positive electrode and the negative electrode, respectively. The electromotive force (EMF) of the battery is determined by the difference in the standard electrode potential (E0) of the redox couple forming the catholyte and the anolyte. On the other hand, where the electrolyte is supplied from an electrolyte tank by a pump, then the redox flow battery has both the advantage of a general battery that has rapid oxidation and reduction reaction speeds on the surfaces of the positive electrode and the negative electrode and the advantage of a fuel cell that has high output power.